Well assembly for heating a subterranean formation



March 12, 1968 E. E. MCDONALD WELL- ASSEMBLY FOR HEATING A SUBTERRANEAN FORMATION Filed May 31, 1966 2 Sheets-Sheet l EVERETTE E. MCDONALD -INVENTO'R ATTORNEY March 12, 1968 E E, MCDONALD 3,372,754

WELL ASSEMBLY FOR HEATING A SUBTERRANEAN FORMATION Filed May 31, 1966 2 Sheets-Sheet 2 I EVERETTE E. MCDONALD INVENTOR ATTORNEY FIG. 4

United States Patent Office 3,372,754 Patented Mar. 12, 1968 3,372,754 WELL ASSEMBLY FOR HEATING A UBTERRANEAN FORMATION Everette E. McDonald, Eagle Pass, Tex, assignor to Mehi1 Oil Corporation, a corporation of New York Filed May 31, 1966, Ser. No. 554,078 8 Claims. (Cl. 166-59) This invention relates to gas-fired heaters for heating a subterranean formation about a well drilled from the surface of the earth. More particularly, the invention pertains to a well assembly for heating an oil-containing subterranean formation to increase the recovery of the liquid petroleum hydrocarbons, more commonly called oil, therefrom.

Oil accumulated within a subterranean formation can be recovered, or produced, through wells from the formation using the natural energy within the formation. However, producing operations deplete the natural energy relatively rapidly. Furthermore, very viscous oils do not readily flow from the subterranean formation. Thus, a large amount of the oil is left in a subterranean formation if only the natural energy is used to produce the oil. Operations to supplement the natural energy in the subterranean formation recover a greater portion of the in situ oil. In subterranean formations containing viscous oil, thermal methods are employed to supplement the natural formation energy.

Two broad thermal methods have been outstanding. In one of the thermal methods, a hot fluid is injected into the subterranean formation to effect a reduction in viscosity of the in situ oil and to supplement the natural formation energy. In another of the thermal methods, combustion is initiated in the subterranean formation. A portion of the oil and the carbonaceous residue are burned in I the formation to generate hot fluids in situ, to supplement the natural formation energy, and to effect the desired high recovery of oil.

Many subterranean formations have erratic distributions of oil. Because of the erratic distribution of oil, they require supplemental heat, at times even after combustion has been started, to continue in-situ combustion. Particularly in these subterranean formations, there is a need for a large heat input.

In thermal recovery methods, various types of heaters have been employed in the past to heat the injected fluid or to provide the heat for initiating and maintaining combustion. Usually, these heaters have suffered from one or more defects. Most of the heaters have had limited heating capacity if they were movable within a well. For example, past electric heaters have a capacity of about 2.5 million B.t.u.s per day. For in situ combusion, many heaters have structural components which limit the spots at which they can be located in a well penetrating the formation to be ignited. For example, the top portion of the heater may have to be maintained at a much lower temperature than the burning formation, requiring that the heater be located at the top of the formation. Consequent- 1y, ignition would start at the top of the formation. Furthermore, such heaters lacked the ability to withstand the heat sustained in traversing the entire formation to obtain a more nearly uniform ignition profile. Many of the heaters were operable for only short periods of time since otherwise they would melt off their supporting member, be it tubing or cable. Some heaters which were gas fired did not burn cleanly. Hence, they deposited particles of carbon on the subterranean formation by their carbon containing flue gases, undesirably reducing permeability. Other heaters required conduits to conduct the flue gases back to the surface. Consequently, the heat contained in the flue gases was lost to the formation. Moreover, the beneficial effects of the carbon dioxide and steam in the flue gases were lost to the formation by not being injected thereinto. Other heaters have fed a combustible mixture into the Well, instead of two separate streams of fuel and combustion-supporting gas, resulting in flashbacks and explosions. Finally, past heaters have employed expensive and difiicultly replaceable components, and were not readily repairable in remote locations.

Accordingly, it is an object of this invention to provide a well assembly, including a heater, which alleviates all of the foregoing disadvantages.

It is a further object of this invention to provide a well assembly having a gas-fired heater which is substantially trouble-free, economical, and mobileboth within a given well and from well to We1lfor heating a subterranean formation.

Further objects and attendant advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic representation illustrating a preferred embodiment of the invention;

FIGURE 2 is a cross section of the heat shield and seating shoulder;

FIGURE 3 is a cross section of the bottom portion of the fuel string; and

FIGURE 4 is a cross section of a resistance heating element being employed as the preferred electrical igniter.

In accordance with the invention, an oil-containing subterranean formation penetrated by a well is heated employing a Well assembly comprising:

(a) Casing within the well extending substantially the entire length of the well from a point in the vicinity of the surface of the earth to Within the vicinity of the formation and communicating with the formation,

(b) Cementing means bonding the casing to the wall of the well, allowing communication between the interior of the casing and the formation but preventing communication between the interior of the casing and other formations,

(c) A casinghead attached to the casing at the upper end thereof,

((1) An aperture having a sealing means and located in the casinghead,

(e) A primary air conduit slidably positioned within the sealing means and the aperture and through the casinghead,

(f) A fuel string concentrically located within the primary air conduit and extending substantially the entire length thereof,

(g) Means preventing leakage of air from the upper end of the primary air conduit,

(h) Means preventing leakage of fuel gas from the upper end of the fuel string,

(i) Means for supplying primary air to the primary air conduit,

(j) Means for supplying fuel gas to the fuel string,

(14) Means for supplying secondary air to the casing,

(l) A heat shield positioned on the primary air conduit at the bottom thereof adjacent the formation,

(In) Means for mixing the primary air and the fuel gas to form a combustible mixture only after reaching the vicinity of the heat shield,

(n) An electrical igniter located within the heat shield and positioned for flow thereover of the fuel gas from the fuel string and of the air conduit, and

(0) Electrical conductor means extending from the surface of the earth through the fuel string to provide electrical energy to the electrical igniter.

By employing the well assembly of the invention, the subterranean formation may be thermally stimulated by the injection of heated fluids thereinto and subsequently primary air, from the primary producing the fluids out of the same well. Alternatively, by employing the well assembly, the subterranean formation can be heated to temperatures sulficient to initiate in-situ combustion therein. Furthermore, adequate heat can be generated employing the well assembly to sustain in-situ combustion even in subterranean formations containing erratic distribution of oil therein.

In employing the invention, the subassembly which is slidable within the sealing means located in the casinghead on the well penetrating the subterranean formation is especially useful. The subassembly comprises:

(a) A primary air conduit,

(b) A fuel string located within the primary air conduit and extending substantially the entire length thereof,

Means preventing leakage of air from the upper end of the primary air conduit,

(d) Means preventing leakage of fuel gas from the upper end of the fuel string,

(e) Means incorporating flexible conduit for supplying primary air to the primary air conduit,

(f) Means incorporating flexible conduit for supplying fuel gas to the fuel string,

(g) A heat shield positioned on the primary air conduit at the bottom thereof adjacent the formation,

(h) Means for mixing the primary air and the fuel gas to form a combustible mixture only after reaching the vicinity of the heat shield,

(i) An electrical igniter located within the heat shield and positioned for flow thereover of the fuel gas from the fuel string and of the primary air from the primary air conduit,

(j) Electrical conductor means extending from the surface of the earth through the fuel string to provide electrical energy to the electrical igniter, and

(k) A hoisting yoke connected to the upper end of the primary air conduit above the casinghead.

Referring to FIGURE 1, well penetrates subterranean formations 12 from the earths surface 14 to an oil-containing subterranean formation 16. Casing 18 lines the well throughout substantially its entire length. Particularly, casing 18 runs from a point in the vicinity of the earths surface to a point in the vicinity of the oilcontaining subterranean formation 16. Preferably, casing 18 extends into the oil-containing subterranean formation and has perforations 20 through which fluids can flow between the interior of the casing and subterranean formation 16. Cementing means 22 bonds the casing to the wall of the well and prevents the fluids from flowing into subterranean formations 12 other than the oil-containing subterranean formation 16. Cementing means 22 may be conventional cement. Preferably, however, at least the bottom portion of cementing means 22 is an insulating cement such as calcium aluminate cement containing about 60 percent by weight of silica flour to prevent high temperature strength retrogression. The percent by weight is based on the weight of the dry calcium aluminate cement. The usual amount of water affording a pumpable slurry and a substantially homogeneous set cement is employed in the insulating cement.

Casinghead 24 is attached to the upper end of casing 18. Casinghead 24 contains aperture 26 having a sealing means (not shown) around its periphery. A suitable sealing means is simply an O-ring. Casingheads containing the aperture and the sealing means are commercially available, e.g., the Hinderliter Casinghead.

Primary air conduit 28 is slidably positioned within aperture 26 and within the sealing means. The primary air conduit 28 thus may be raised or lowered without leakage through the casinghead from the interior of the casing to the atmosphere. Further, it traverses the entire length of the well from the surface 14 to the oil-containing subterranean formation 16. The amount of distance which it extends above the casinghead depends upon whether the heater is to be raised or lowered in subsequent operations, as explained more fully hereinafter.

Fuel string 30 is placed concentrically within the primary air conduit 23. Fuel string 39 traverses substantially the entire length of the primary air conduit. It is sealed to the upper end of the primary air conduit by suitable means, such as concentric bushing 32, preventing leakage of primary air to the atmosphere. It terminates in a lubricator 34 serving as means preventing leakage of fuel gas to the atmosphere. The lubricator is an extension of fuel string 30 having a sealing valve 36 and an access cap 38 to permit entry into the conduit under operating conditions. Access cap 38 contains a concentric opening, compressing bushing, and soft seal 39 to allow a lineal member to be inserted therethrough yet not leak fluids rorn the interior to the atmosphere. Conduit 4t and flexible conduit 42 connect the primary air conduit with an air compressor (not shown) or other suitable source of air and comprise means for supplying air, hereinafter called primary air, to the primary air conduit. Valve 44 and meter 46 may be incorporated in conduit 40 to control the flow of primary air. Flexible conduit 42 may comprise flexible hoses or conventional pipe employing Chick-San joints. This conduit 42 enables the primary air conduit to be raised or lowered within the well without repiping.

Similarly, conduit 48 and flexible conduit 56 connect the fuel string with a suitable supply of fuel gas (not shown) and comprise means for supplying fuel gas to said fuel string. Conduit 48 may incorporate a valve $2 and positive displacement meter 54 for control of the flow of fuel gas to the fuel string.

Conduit 56 connects the casing interior with a suitable source of air, which may he the air compressor supplying the primary air to the primary air conduit. This conduit 56 may incorporate valve 58 and meter 6t} for control of secondary air flowing into the casing. The term secondary air is employed to denote air used as a convection means to carry heat away from the primary air conduit 28 and heat shield 62 and into the oil-containing subterranean formation Tu and to support secondary combustion Where appropriate, e.g., burning the in situ oil. Thus, the secondary air is not employed directly as combustion-supportin air to burn the fuel gas in the combustion chamber i? within the heat shield 62.

The details of construction following are also illustrated in FIGURE 2, FIGURE 3, and FIGURE 4, which are described hereinafter. Heat shield 62 is positioned on the primary air conduit 28 at its bottom end and adjacent the formation. The primary air conduit serves as an inner steel conduit 63. Seating shoulder 64 is concentrically fixed within the heat shield and on the interior of the inner steel conduit. Fuel string 30 seats upon seating shoulder 6 blocking the flow of primary air at this point. Located an appropriate distance above the end of fuel string 30, primary air entry slots 66 afford means for the primary air to enter into the fuel string and mix with the fuel gas as it flows past electrical igniter 6S and into the combustion chamber 67.

Fuel string 3t) contains igniter assembly seat 70 attached and arranged concentrically interiorly.

Seating collar "72 is concentrically attached to the exterior of case 73 of the igniter assembly 7 4. To start combustion, seating collar 72 seats against igniter assembly seat 7t) and forces the fuel gas to flow down the interior of igniter assembly 74. Gas bypass slot 7c enables the fuel gas to mix with the primary air to form a combustible mixture. The combustible mixture then flows over and in contact with the electrical igniter 6%.

Electrical igniter 68 is located preferably at the bottom extremity of igniter assembly '74. Multiple vent holes 39 induce turbulence and allow thorough mixing of the fuel gas and primary air. Electrical conductor means in the form of cable 82, which may be inside tensile cable 83, connects electrical igniter 63 to a power source 84 at the surface. Case '73 of igniter assembly 7 3, fuel string 3%, and. lubricate: provide a ground-conducting means since electrical igniter 63 is connected to the conducting case of igniter assembly 74. Ground wire 86 connects lubricator 34 with power source 84 to complete the electrical conductor means, i.e., the circuit. Power source 84 may be any suitable power plant or auxiliary power package containing the requisite generator and switch facilities. Power source 84 supplies electrical power through the electrical conductor means to heat the electrical igniter 68.

A spark plug may be employed as the igniter if desired. A suitable ignition system incorporating a high voltage transformer serves as the power source in that event. The proportion of fuel gas and primary air in the combustible mixture must be controlled more accurately to obtain successful ignition with a spark plug than with a resistance heating element. Therefore, resistance heatingelement '78 is illustrated as the preferred essential element of igniter 68.

Thermocouple 87 is positioned in the well near heat shield 62. Thermocouple conductor 88 connects the thermocouple 87 to a temperature transdncing means 89 at the surface 14. The temperature transducing means 89 affords an indication of the temperature in the environment in which the thermocouple has been positioned. The thermocouple may be inserted through the fuel string to a point in the vicinity of the heat shield. Preferably, the thermocouple is positioned in the annular space between the heat shield and the casing and at the lower portion of the heat shield. When thus positioned, the thermocouple more readily senses when burning starts.

In operation, fuel gas from the fuel gas supply flows via conduits 48 and 50 into fuel string 30. Simultaneously, air flows from an air source, such as an air compressor, through conduits 40 and 42 into primary air conduit 28;. The air flows through primary air entry slots 66, and the fuel gas flows through gas bypass slot 76, all inside heat shield 62. Only then does the fuel gas and air mix to form a combustible mixture, and the combustible mixture flows over resistance heating element 78 where it is heated to its ignition point. The combustible mixture burns in the combustion chamber 67 inside heat shield 62 and in the well adjacent the oil-containing subterranean formation.

Thermocouple 87 signals, via thermocouple conductor 88 to temperature transducing means 89 at the surface, the increase in temperature accompanying the burning of the combustible mixture. After the increase in temperature has been noted, indicating combustion has been initiated, igniter assembly 74 is raised within fuel string 30 to a point where lower temperatures prevail. Igniter assembly 74 may be raised with tensile cable 83.

The primary air continues to flow down primary air conduit 28. Fuel gas continues to flow down fuel string 30. The primary air flows through primary air entry slots 66, mixing with the gas and sustaining the combustion in the combustion chamber 67.

Secondary air flows from the air source through conduit 56, down casing 18, and into the oil-containing subterranean formation 16. The secondary air cools the primary air conduit and other well equipment, carrying the heat by convection into the subterranean formation. As noted, the secondary air is not employed directly in sustaining combustion of the fuel gas. However, it does contain oxygen and affords a combustion-supporting gas for secondary heating. The secondary heating may be limited to oxidizing the carbon monoxide in the flue gases. Usually, however, and particularly in situ combustion, the secondary air enables ignition of the oil and the carbonaceous residue within the subterranean formation 16.

' The -wellhead assembly allows supplying up to 50 million, ormore, B.t,uls per day to a subterranean formation. The hot flue gases containing beneficial carbon dioxide fiow into the'oil containing subterranean formation, affording the well-known benefits attendant to carbon dioxide absorption into the in-situ oil. Further, the hot flue gases contain steam to afford the well-known 16 and ensure a more nearly uniform ignition over the entire thickness of the formation. With this initial positioning, the primary air conduit need not extend any significant distance above casinghead 24, as previously mentioned. A hoisting yoke 98 is attached to the upper end of primary air conduit 28. A manual hoist (not shown) may be suspended from an A-frame (not shown) and attached to hoisting yoke 99 for manually raising the primary air conduit while the burner is in operation. For deep wells, a pulling unit or power hoist may he required to raise the primary air conduit.

infrequently, it may be desirable to ignite the top of the subterranean formation first and move the burner assembly downward within the formation. For such operating technique, the burner assembly is positioned initially at the top of the subterranean formation. With this initial positioning, the primary air conduit may extend above casinghead 24 a distance approximating the thickness of the subterranean formation. Ordinarily, however, the primary air conduit will not extend much more than the length of one joint of tubing above the casinghead at any one time. In some instances, the weight of the slidable portion of the well assembly is inadequate to overcome the operating pressures tending to force it from the well. In the event that both of these circumstances are encountered, hoisting yoke must have members capable of bearing compression. Additionally, the pulling unit whichis employed to lower the primary air conduit and the remainder of the slidable portion of the well assembly must have a means of forcing the burner assembly downward against the operating pressure.

For traversing thicker formations to obtain more nearly uniform heating, additional joints of tubing may be inserted in or removed from, as the case may be, the primary air conduit and the fuel string with only short interruptions in heating operations.

The details of construction of the heat shield are shown in FIGURE 2. Primary air conduit 28 has seating shoulder 64 attached and arranged concentrically interiorly to form inner conduit 63. Inner conduit 63 is concentrically positioned within outer conduit 94. A refractory insulating cement fills the annular space and bonds the inner conduit comprising primary air conduit 28 and seating shoulder 64 to outer conduit 94. The inner and outer conduits may be stainless steel or other heatresistant alloy. Ordinarily, however, conventional schedule 40-line pipes of appropriate dimensions are adequate. A suitable refractory insulating cement is the previously described calcium aluminate cement with 60 percent by weight silica flour. Alternatively, the calcium aluminate cement with 60 percent by weight silica flour may be mixed in approximately equal parts with ground bricks, sometimes called grog. The usual amount of water, as previously described, is employed in making the slurry to fill the annular space and effect the set refractory insulating cement 95. Outer conduit 94 and annular refractory insulating cement 95 may be as long as needed for the environment in which the heater assembly is to be operated. For example, it may run as short as two to five feet in length. On the other hand, it may run a hundred feet, or more, in length in particularly thick subterranean formations.

The details of construction of the bottom end of the fuel string 30 are shown in FIGURE 3. The bottom end of the fuel string contains the previously described igniter assembly seat 70, and primary air entry slots 66. It also is fixed to a terminal portion 98. Terminal portion 98 is machined to slide within the interior of the portion of the heat shield above seating shoulder 64, but it seats upon seating shoulder 64 to seal off the primary air passage. Consequently, it forces the primary air to flow through primary entry slots in the interior of the fuel string and mix with the fuel gas.

The details of construction of the igniter assembly 74 are shown in FIGURE 4. The case 73 of igniter assembly 74 consists essentially of a small conduit movable within the fuel string and has multiple vent holes 88 and gas bypass slot 76. Support attachment 102 provides a point to which tensile cable 83 can be attached. Further, support attachment 102- provides a point for attaching resistance heating element 78 to cable 32. The other end of resistance heating element 78 is attached to case 73. Around the exterior and at the upper end of the igniter asse-mbly 74 is located seating collar 72 which seats onto igniter assembly seat '79 in fuel string as previously described.

The following example is a particular embodiment which illustrates the invention and its advantages.

EXAMPLE In the shallow Chittim Ranch subterranean formation in Maverick County, Tex, the well assembly described hereinafter was employed to effect in situ combustion. In the Chittim Ranch subterranean formation, the distribution of oil is discontinuous and erratic, and large quantities of heat are required at times to maintain in situ combustion. The well assembly including the gas burner was used on well Y-G for twenty-eight days without damaging the shoe or the casing of the well. In contrast, another gas burner rented and put into a different, but comparable, Chittim Ranch well had burned up a part of the casing and shoe within three days without supplying as much heat per unit time.

The following table describes components found suitable for application in the Chittim Ranch subterranean formation wherein the in situ combustion was carried out.

Table Component: Description Casing 18 4 /2-inch casing slotted in tire oilcontaining subterranean formation.

Casinghead 24 2-inch Hinderliter.

Aperture 2.6 and sealing means 2-inch doughnut-shaped rubber seal.

Primary air d it 23 Z-inch schedule -line pipe- 2.375 inches outside diameter (O.D.), 2.067 inches inner diameter (I.D.).

Fuel string 30 l-inch schedule 40-line pipe- 1.315 inches O.D., 1.049 inches I.D.

Lubricator 34 2-inch schedule 40-line pipe with screwed access cap.

Conduit 40 2-inch schedule 40-line pipe.

Conduit 48 l-inch schedule 40-line pipe.

Flexible conduit 42 2-inch schedule 40-line pipe with Chick-San joints. Flexible conduit 5% l-inch schedule 40-line pipe with Chick-San joints. Meters for air Orifice meters. Meters for gas Positive displacement meters.

8 Component-Continued Description Outer conduit 94 of heat shield 3-inch schedule 40-line pipe -3.5

inches O.D.

Refractor insulating cement Calcium aluminate cement with 60 percent silica flour. Terminal portion 98 l-inch collar machined to 1.45

Resistance heating element 78 15 -arnp.l l0-voltage design, insulated within igniter assembly by ceramic insulators.

The primary air conduit was raised a maximum distance equal to the length of one joint of tubing by a hand hoist attached to an A-frame and a hoisting yoke. Temperatures were measured inside the gas string. Combustion started early. Temperatures up to 2000 F. were measured for the major part of the twenty-eight-day period.

Combustion was initiated employing a 10 percent by volume fuel gas and percent by volume primary air mixture across resistance heating element 78. Following ignition, the percentage fuel gas in the combustible mixture was lowered to 2.5 percent by volume. The igniter assembly was left in the combustion zone for up to thirty-five minutes after ignition without undue damage thereto.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A well assembly for effecting heating over portions of a subterranean formation penetrated by a well comprising: v

(a) a casing within said well extending substantially the entire length of said well from a point in the vicinity of the surface of the earth to within the vicinity of said formation and communicating with said formation,

(b) cementing means bonding said casing to the wall of said well, allowing communication between the interior of said casing and said formation but preventing communication between said interior of said casing and other formations,

(c) a casinghead attached to said casing at the upper end thereof,

(d) an aperture having sealing-means in said casing head,

(e) a primary air conduit slidably positioned within said sealing means and said aperture in said casinghead,

'(f) a fuel string concentrically located within said primary air conduit and extending substantially the entire length thereof, t

(g) means preventing leakage of air fromthe upper end of said primary air conduit,

(b) means preventing leakage'of fuel gas from the upper end of said fuel string,

(i) means for supplying primary air to said primary air conduit,

(j) means for supplying fuel gas to said fuel string,

(k) means for supplying secondary air to said casing,

(l) a heat shield positioned on said primary air conduit at the bottom thereof adjacent said formation,

(in) means for mixing said primary air and said fuel gas to form a combustible mixture only after reaching the Vicinity of said heat shield,

(11) an electrical igniter located within said heat shield and positioned for flow thereover of fuel gas from said fuel string and of air from said primary air conduit, and

() electrical conductor means extending from the surface of the earth through said fuel string to provide electrical energy to said electrical igniter.

2. The well assembly of claim 1 wherein said primary air conduit has a hositing yoke connected to its upper end above said casinghead.

3. The well assembly of claim 2 wherein said means for supplying air to said primary air conduit incorporates a flexible conduit and said means for supplying fuel gas to said fuel string incorporates a flexible conduit.

4. The well assembly of claim 1 wherein a thermocouple is positioned adjacent said heat shield, a temperature transducing means is positioned at said surface, and thermocouple conductor means connects said thermocouple with said temperature transducing means to measure the temperature in said well.

5. The well assembly of claim 1 wherein said heat shield comprises an inner steel conduit and an outer steel conduit concentrically arranged, and a set cement made from calcium aluminate cement with 60 percent by weight of said cement of silica flour or from said calcium aluminate cement and particles of ground brick mixed in approximately equal proportions within the annular space separating said concentrically arranged inner steel conduit and outer steel conduit, said inner steel conduit containing a seating shoulder fixed concentrically interiorly.

6. The Well assembly of claim 1 wherein:

(a) said casing extends from a point in the vicinity of the surface of the earth to within the vicinity of the bottom of said formation and contains perforations at its lower end to permit heated fluids to pass from said casing to said formation,

(b) said primary air conduit contains a seating shoulder,

(c) said gas string seats on said seating shoulder, contains interior igniter assembly seat, and contains primary air entry slots below said igniter assembly seat whereby primary air flows from said primary air conduit into the interior of said fuel string, and

(d) said igniter comprises a resistance heating element within a small diameter conduit having an exterior seating collar and containing a gas bypass slot and perforations whereby said primary air and said gas can mix and flow over said resistance heating element for ignition.

7. The well assembly of claim 1 wherein said electrical conductor means is connected to a power source supplying electrical energy and located at the surface of the earth.

8. A subassembly which is slidable within a sealing means located in a casinghead on a Well penetrating a subterranean formation, comprising:

(a) a primary air conduit,

(b) a fuel string located within said primary air conduit and extending substantially the entire length thereof,

(c) means preventing leakage of air from the upper end of said primary air conduit,

(d) means preventing leakage of fuel gas from the upper end of said fuel string,

(e) means incorporating flexible conduit for supplying primary air to said primary air conduit,

(f) means incorporating flexible conduit for supplying fuel gas to said fuel string,

(g) a heat shield positioned on said primary air conduit at the bottom thereof adjacent said formation,

(b) means for mixing said primary air and said fuel gas to form a combustible mixture only after reaching the vicinity of said heat shield,

(i) an electrical igniter located within said heat shield and positioned for flow thereover of said fuel gas from said fuel string and of said primary air from said primary air conduit,

(j) electrical conductor means extending from the surface of the earth through said fuel string to provide electrical energy to said electrical igniter, and

(k) a hoisting yoke connected to the upper end of said primary air conduit above said casinghead.

References Cited UNITED STATES PATENTS 2,506,853 5/1950 Berg et a1 166-59 2,584,606 2/ 1952 Merriam et al 166-59 X 2,668,592 2/1954 Piros et a1. 166-59 X 2,685,930 8/1954 Albaugh 166-59 X 2,832,417 4/1958 Ford 1 66-59 2,877,847 3/1959 Pelzer et al. 166-39 2,941,595 6/1960 Emery 166-39 X 3,004,603 10/1961 Rogers et a1 166-39 X 3,298,439 1/1967 Rees 166-59 3,307,609 3/1967 Hujsak 166-59 X 3,315,745 4/1967 Rees 166-59 STEPHEN J. NOVOSAD, Primary Examiner. 

1. A WELL ASSEMBLY FOR EFFECTING HEATING OVER PORTIONS OF A SUBTERRANEAN FORMATION PENETRATED BY A WELL COMPRISING: (A) A CASING WITHIN SAID WELL EXTENDING SUBSTANTIALLY THE ENTIRE LENGTH OF SAID WELL FROM A POINT IN THE VICINITY OF THE SURFACE OF THE EARTH TO WITHIN THE VICINITY OF SAID FORMATION AND COMMUNICATING WITH SAID FORMATION, (B) CEMENTING MEANS BONDING SAID CASING TO THE WALL OF SAID WELL, ALLOWING COMMUNICATION BETWEEN THE INTERIOR OF SAID CASING AND SAID FORMATION BUT PREVENTING COMMUNICATION BETWEEN SAID INTERIOR OF SAID CASING AND OTHER FORMATIONS, (C) A CASINGHEAD ATTACHED TO SAID CASING AT THE UPPER END THEREOF, (D) AN APERTURE HAVING SEALING MEANS IN SAID CASING HEAD, (E) A PRIMARY AIR CONDUIT SLIDABLY POSITIONED WITHIN SAID SEALING MEANS AND SAID APERTURE IN SAID CASINGHEAD, (F) A FUEL STRING CONCENTRICALLY LOCATED WITHIN SAID PRIMARY AIR CONDUIT AND EXTENDING SUBTANTIALLY THE ENTIRE LENGTH THEREOF, (G) MEANS PREVENTING LEAKAGE OF AIR FROM THE UPPER END OF SAID PRIMARY AIR CONDUIT, (H) MEANS PREVENTING LEAKAGE OF FUEL GAS FROM THE UPPER END OF SAID FULE STRING, (I) MEANS FOR SUPPLYING PRIMARY AIR TO SAID PRIMARY AIR CONDUIT, (J) MEANS FOR SUPPLYING FUEL GAS TO SAID FUEL STRING, (K) MEANS FOR SUPPLYING SECONDARY AIR TO SAID CASING, (L) A HEAT SHIELD POSITIONED ON SAID PRIMARY AIR CONDUIT AT THE BOTTOM THEREOF ADJACENT SAID FORMATION, 